GB2122455A - Communications systems - Google Patents

Description

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SPECIFICATION

Communications systems The present invention relates to communication systems.

According to the invention, there is provided a communications system, comprising a plurality of two-way radios connected to a junction unit, and a plurality of operating terminals interconnected to each other and to the junction unit in a cable circuit, each operating terminal including meansforfeeding communication signals onto, and receiving communication signals from the cable circuit, the junction unit being operative to generate repeated cycles each defining n notional frames which are in turn each divided into a plurality of time slots which is the same for each frame, at least some of the time slots in each frame defining different communication channels which respectively permit communication between a particularone, only, of the radios and any one or more of the operating terminals.

The invention also provides a radio harness for a vehicle comprising a plurality of operating terminals and a plurality of radiosfor external communication interconnected by cable means, and meansfor generating a time-div[sion-multiplexing frame structure such that respective, recurring time slots are provided within said frame structure forallowing the operating terminals to receive and transmit via each radio.

A communications system embodying the invention will now be described, byway of example, with reference to the accompanying diagrammatic drawingsinwhich:

Figure 1 shows a block circuit diagram of the system; Figure 2 is a schematic circuit diagram of a channel switching arrangement of the system of Figure 1; Figure 3 is a timing chart illustrating the division of the system signals into separatetime slots; Figure 4 is a block diagram of a radio junction box of the system of Figure 1; Figure 5 is a block diagram of a production circuit of the radio junction box of Figure 4; Figure 6 is a block diagram of a crew box in the system; and Figure7 is acircuit diagram of the clamping circuit of the crew box of Figure 6.

The communications system to be described is arranged to be used in a vehicle manned by a crew and carrying, for example, three two-way radios for communication between the vehicle and stations remote therefrom. The system provides each crew- man with an adjacent crew box orterminal at his working position and these are linked to a junction box and thence to the three radios, so that each crewman can communicate through any of the three radios with each of the remote stations. He can also call and communicate with his fellow crewmen over an internal intercom channel. In the system to be described, each of the three radios is allocated a separate time multiplexed channel. While any of these channels is selected, however, any one or more of the crewmen can transmit or receive from the GB 2 122 455 A 1 selected radio. Similarly,the intercom facility is allocated a fourth time multiplexed channel and when this is selected, any one or more of the crewmen can transmit or receive. In otherwords, the system does not require individual time multiplexed channels for each of the crew boxes. The manner in which this is achieved and yet requiring only a simple send and return cable connecting all the crew boxes and the junction box will be explained.

The system shown in Figure 1 includes a radio junction box or unit 2 which is coupled to three two-way radios, Radio A, Radio B and Radio C, and two crew boxes 4 and 6. The three boxes 2 to 6 are linked by a line 8 consisting of a single coaxial cable 8, the central conductor being the signal transmission path and positive supply, and the outer screen acting as a ground return path. The coaxial cable 8 used can be regarded as a transmission line with matched impedances Z of, say, 50 ohms at opposite ends.

Further crew boxes can be coupled to the line 8 as required withoutthe need of significant alteration to the systems as will be explained.

The radio junction box includes a generatorfor feeding a D.C. supply currentwhich is fed through the line 8to each of the crew boxesto energise the circuitry in each of them. A constant current circuit, to be described below, in each crew box ensures that each crew box draws a constant current from the line 8 irrespective of its operative state. The junction box 2 also has a sync pu Ise generator which generates successive trains of sync pulses that are placed on the line 8. Advantageously the line voltage is 18 volts and the signal voltage may be of the order of 250 mV. Each train of sync pulses defines a series of time slots, each time slot being allocated to a specific communication channel in a mannerto be explained. Certain communication channels are for digital data for controlling the system while others are for analogue signals such as audio signals as will now be described in more detail.

As shown in Figure 2, each radio has three terminals. Thus Radio A has an earphone terminal A,, a microphone terminal A2 and a press to talkterminal A3for switching the radio between transmit and receive states. Radios B and C have similar sets of terminals B, to B3 and C1 to C3- Each crewman's box has twelve sets of terminals, nine terminals A,, A2, A3, B1, B2, B3, C1, C2, C3 corresponding with respective ones of thethree sets of terminals of the three radios and a furtherthree terminals D, to D3 associated with the common intercom. Terminal D, is the earphone terminal, D2 is the microphone terminal and D3 is the call terminal of the intercom.

Each of the crew boxes and the radio junction box incorporates an electric switch for connecting each terminal A, to D3 inturn tothe line 8. Figure 2 illustrates this in terms of physical switches while in practice the channels are selected by a gating circuit triggered by appropriate gating pulses. By synch ronising the operation of the switches, only similarly referenced terminals will be interconnected during each time slot and in this way be selecting a particular terminal (that is a pa rticula r time slot in each train of sync pulses) a crewman can select a particular 2 GB 2 122 455 A 2 communications channel.

Thus,for example, by selecting terminals A, to A3 and coupling them respectivelyto his earphones, microphone and press to talk switch, a crewman can communicate with an outside station through Radio A.

During this time, for example, another crewman can communicate through Radio B,oreven on Radio A (by sharing the channel with the first crewman).

This multi-channel communication system is in fact achieved by a time division multiplexing process which will now be described.

A shown in Figure 3, the synch ronising pulses define complete cycles of the time division multiplex- ing process. Each cycle is (in one example), 400 tLS long and divided from the next cycle by a "block sync" pulse of greater length than the othersync pulses. Each cycle is notionally divided into four frames, each 1 OOILS long and each frame in turn is divided into ten time slots, with the beginning of each time slot being indicated by a respective sync pulse. The first, ninth and tenth slots (S1, S9 and S1 0) in each frame are allocated to digital control information while the second (S2) to the eighth (S8) time slots are allocated to analogue information. More specifically the second time slot S2 in each frame is allocated an intercom channel during which the crew boxes can communicate with each other. Thethird, fifth and seventh time slots S3, S5, S7 are allocated to receive signalsfrom Radios Ato C respectively, while the fourth, sixth and eighth time slots S4, S6, S7 are allocated to transmit signals to thethree Radios Ato C respectively. The first time slot S 1 in Frame 1 carries the block sync pulse which isused in a mannerto be described to synch ronise all the crew boxes and the ninth time slot S9 in this frame is used to transmit a "CALU'signal to all the crew boxes for the intercom channel. In Frame 2, the firsttime slot S1 isspare and the nintltime slot S9 is for carrying a D.C. control level forthe press to talk signal for Radio A. In Frame 3,thefirstslot S1 is spare and the ninth time slot S1 carries a D.C. control level forthe press to talk signal for Radio B. In Frame 4, slot S1 is spare and slot S9 carries a corresponding D.C. level for Radio C. In each framethetenth time slot S1 0 is spare. Slots S1 and S1 0 may be used for re-broadcasting purposes as will be exiained.

In Figure 3, the third frame is shown expanded with slot S2 allocated to the internal channel (1c), slot 3 allocated to receive signals from Radio A (Ain), slot 4 allocated to transmit signals to Radio A (A,,t), slots 5 to 8 allocated to Radios B and C in similarfashion, and slot 9 allocated to the pressto talk signal (P-T) for Radio B. Also shown in Fig. 3 is a diagrammatic enlargement of slot S5 showing how its start is defined by a sync pulse P1 and its end by the following sync pulse P2. The analogue information transmitted in thetime slot is shown diagrammaticallyatAS.

In thejunction box 2, the sync pulses generated by a sync pulse generator 42 are coupled to the coaxial line 8through a predistrotion transconductance amplifier44, see Fig. 4. A power source 46feeds current onto the coaxial line through an inductor 40.

The inductor 40 prevents the power supply from loading and distorting the audio signals on the line 8. In practice, however, physical space limitations may make it impossible to use an inductor sufficiently large to prevent ail distortion andtherefore both the sync pulses and the audio signals maysuffersome distortion, particularly at lowfrequencies. In orderto reduce the effect of this distortion on the sync pulses, the predistortion circuit 44 acts to distort each sync pulse by an amountto compensate forthe distortion that itwill sufferon the line 8 before it reaches a crew box. In this way each crew box receives a substantially undistorted signal.

As shown in Figure 5,the predistortion circuit44 includes a differential amplifier 50. One inputtermin- al receives a signal Vin from the sync pulse generator 42 while the other inputterminal is coupled to the emitter of a transistor 51 through a feedback impedance 52. Thevoltage across impedance 52 determinesthe output current lo. If the transfer function 0 of the feedback loop is made the same as the transfer impedance of the line 8, the transfer conductance of circuit44will be -1/P.

Vi.

Thevoltageonthe line8wilithen beV,=1o.por Vc=Vin, that is, the undistorted signal generated by the generator 42.

The circuitry of each crew box is shown in Figure 6. Powerforthe crew box istaken off the line 8 by a currenttake-off device 54which feeds a fixed output current (e.g. of, say, 100 mA) to the units of the crew box (the connections to the units are indicated merely bythe arrowX and are individually omitted for clarity).

A shunt regulator unit 55 ensures that the fixed current (of 100 mA in this example) is always taken by the currenttake-off device 54. That is, if the units of the crew box are consuming less than 100 mA, the regulator 55 shunts more currentto ground (i.e. the return path of the coaxial cable) to compensate.

In the crew box, a 20 dB buffer amplifier 60 receives audio signaisfrom the line 8 and feeds them through a clamping circuit 62to a 10 dB buffer amplifier 64. The amplifier 64feedstwo switching circuits 66 and 68 in parallel. Each switching circuit 66 and 68 feeds a respective earphone 78,80 of a set of earphones through a corresponding filter70,72 and a corresponding amplifier74,76. Each amplifier74,76 is providedwith a volume control adjustment facility.

Async pulse detector82 is also coupled tothe output of amplifier60 and feeds sync pulses to a central control unit84. The central control unit includes counterswhich are resetatthe beginning of each cycle (see Fig. 3) bythe blocksync pulse and then couritthe subsequentsync pulseswhich define thetimeslots of the cycle. The central control unit84 is therefore able to emit control signalsto the various units of the crew box at the appropriate time instants as will be explained.

A balanced microphone 86 drives a microphone amplifier 88 which in turn feeds a gating circuit 90.

The gating circuit 90 which is controlled by the central control unit 84 drives a differential amplifier 92 which 3 a in turn drives the base of a transistor 94. The collector of the transistor 94 is connected to the coaxial cable 8, while the emitter is connected to ground through a resistor 98 and to the other input of the amplifier through a feedback circuit 96 having unity gain with compensation.

The arrangement of the amplifier 92, transistor 94 and feedback circuit 96 distorts the signal from the microphone by an amount equal to the distortion that the signal will' undergo along the line (and as caused by inductor40 as explained above in connection with Fig. 4) before it reaches a radio or another crew box. As a result, the signal will be received in a substantially undistorted state.The predistortion is effected in a mannersimilarto that described in conjunction with Figure 5. The transistor94 and its associated components therefore act as a current generatorto place on the line 8 a current level dependent on the audio signal produced bythe microphone 86.

The crew box has a two-line control channel Cl and control lines C2, C3, connected to controls by means of which the crewman can select a desired operating mode. Thus, by means of channel Cl, the crewman can select intercom- (enabling him to communicate with the other crew boxes),---RadioA-,---RadioB" or ---RadioC- (enabling him to transmit or receive via the selected radio). Channel Cl therefore energisesthe control unit84 so that itgates outthe appropriate enabling signals during the appropriate time slots.

Thus, for example, if the crewman selects Radio A by means of channel Cl, the control unit 84 enables the switching circuits 66 and 68 du ring time slot S2 (see Fig. 3) so as to pass the audio signals to the earphones 78 and 80, and enables gating circuit 90 during time slot S4 so as to pass the audio signals to the line 8 from the microphone 86. Line C2 is the 11 press to talk" line by means of which the control unit 84 can be caused to feed the appropriate D.C. level on line 8 during time slotS9 of Frame 2 so asto activate RadioA.LineC3isaCALL1inebymeansofwhichthe crewman can send a---call- signal to the other crew boxes via the intercom channel, and the control unit 84 therefore places the appropriate D.C. level on the line8 during time slotS9 of Frame 1.

The output from the buffer amplifier 64 also feeds a call signal detector 100 which detects a "call" signal when generated on the line 8 by another crew box. When such a call signal is detected, the detector 100 sends a signal to the central control unit 84 which feeds enabling signals to the amplifier 76 to switch the amplifier to full volume thus over-riding any setting of the volume control. As a resu It the intercom channel is received in the right earphone 80.

The clamping circuit 62 is shown in more detail in Figure7.

The clamping circuit includes a capacitor 110 interconnecting the two buffer amplifiers 60 and 64 of Fig. 6. The junction between the capacitor 100 and the buffer amplifier 64 is connected through the main current path of a field effecttransistorto ground. The gate electrode of the field effect transistor is connected to the central control unit 84to receive appropriately timed gating pulses therefrom.

In operation, the central control unit84producesa "clamp enable" signal on line 102 to the gate of the GB 2 122 455 A 3 FET 112 in response to the leading edge of each sync pulse. This switches the FET on and sets the D.C level of capacitor 1 10to a known reference (e.g. ground). Afterthe end ofthesyncpulse, but before receipt of anysignal during that time slot, the control unit84 switchesthe FEToff andthe capacitor 110 istherefore ableto actas a coupling capacitorand passthesignal received during thattime slotto the amplifier 64; Fig. 3 shows at tthe time constant when the FETis switched off in slot 35 (and it is switched off at corresponding time instants in the othertime slots). The effect of the clamping action, therefore, is that any offset due to preceding signals is minimised. Over one time slotthe only signal to passto the amplifier 64 is the information associated with the time slot, devoid of any sync pulse.

From the foregoing, itwill be apparentthat signals are placed on the line 8 as currents via current generators atthe crew boxes and in thejunction box.

Each current therefore generates an appropriate voltage signal acrossthe impedance of thecable which is assumed to be constant. Thus, when more than one crew boxfeeds currentontothe line 8,the currents add and generate a correspondingly greater go voltage signal on the line. In otherwords,the attenuating effectwhich would take place if each crew box outputwere in theform of a voltage (in such a case each outputwould be loaded by the relatively low output impedance of the othervoltage output or outputs on the line) is avoided. Time multiplexing of the crew boxes is therefore not required.

It will be appreciated that the spare channels in the frames can be used for many purposes. One application is for controlling rebroadcast facilities. Thus one of the three radios can be selected to receive a signal from one remote station while another radio can be selected to rebroadcastthe signal to another remote station. By applying appropriate digital signalsto the spare channels, selection of the rebroadcast facility can be made from any outside station or any crew box with the aid of an additional rebroadcast unit cou pled to the radio junction box. In such a case, signals in Frame B would be used to select Radio A as the rebroadcasting radio, and signals in Frames C and D would be used to select Radios B and C for rebroadcasting respectively.

The present Application is divided outfrom our co-pending British Application No. 8029388 (Publication No. 2063625A),which claims otheraspects of the

Claims (8)

above described communications system. CLAIMS

1. A communications system, comprising a plurality of two-way radios connected to a junction unit, and a plurality of operating terminals interconnected to each other and to the junction unit in a cable circuit, each operating terminal including meansforfeeding communication signals onto, and receiving communication signals from the cable circuit, the junction unit being operative to generate repeated cycles each defining n notional frames which are in turn divided into a plurality of time slotswhich is the same for each frame, at least some of the time slots in each frame defining different communication channels which respectively permit communication between a par- ticular one, only, of the radios and any one or more of 4 the operating terminals.

2. A system according to claim 1, in which the corresponding communication channels in each frame are defined bythe same serially numbered 5 time slots in the frame.

3. A system according to claim 1 or 2, in which other time slots in each frame define respective channels for passing control information between the radios, the operating terminals and the junction box.

4. Asystem according to anyone of claims 1 to 3, in which each frame includes onetime slot defining an intercom communications channel permitting communication between the operating terminals.

5. Asystem according to anyone of claims 1 to 4, in which the junction box includes means for generating synchronising pulses defining the time slots and forfeeding them to the operating terminals on the cable circuit.

6. Asystem according to claim 5, in which the junction unit includes a pre-distorting circuit for pre-distorting each synch ron ising pulse by an amount substantially equal and opposite to the distortion that it will suffer on the cable circuit.

7. Asystern according to anyone of claims 1 to 6, in which the cable circuit is a single coaxial line.

8. A radio harness for a vehicle comprising a plurality of operating terminals and a plurality of radios for external communication interconnected by cable means, and means for generating a timedivision-multiplexing frame structure such that respective, recurring time slots are provided within said frame structurefor allowing the operating terminals to receive and transmitvia each radio.

Printed for Her Majesty's Stationery Office byTheTweeddale Press Ltd., Berwick-upon-Tweed, 1983. Published atthe PatentOffice, 25Southampton Buildings, London WC2A lAYfrom which copies may be obtained.

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